1. Field of the Invention
The present invention relates to stripping tools for removing insulation from coaxial cables prior to the attachment of an electrical connector.
2. Description of Related Art
Coaxial cables are widely used to distribute many different kinds of signals, such as video and data signals, and come in a wide variety of designs and sizes. The most common type of coaxial cable includes a center conductor surrounded by an inner layer of insulation and an outer conductor protected with an outer plastic jacket.
However, there are many different cable designs and many different types of materials that may be used for the conductors, the insulation and the jacket. The center conductor is most often a solid wire conductor, such as copper or aluminum. However, the outer conductor may be a wire braid, a metallic foil or a metallized tape such as a mylar or other metallized plastic film. The outer conductor may also be constructed with up to four or more different layers in various combinations of braided wire, metallic foil, metallized plastic or other conductive tape material. There may also be various layers of separating material or adhesives used to bond layers together.
Some combinations of these materials result in a cable that is difficult for a conventional stripping tool to prepare for the attachment of an electrical connector. The particular type of cable must be prepared to match the requirements of the selected type of connector. Typically, cable preparation involves removing sections of the insulation and jacket to expose the inner and outer conductors and produce a xe2x80x9csteppedxe2x80x9d cable end. The jacket, outer conductor and insulation are usually removed for a first predetermined distance to expose the inner conductor and the jacket is removed for a second predetermined distance to expose the outer conductor. The first and second predetermined distances depend on the particular type of connector to be attached.
Coaxial cable stripping tools are specifically designed for particular cables and for particular types of connectors to remove the desired amount of cable material efficiently and accurately, without damaging the inner and outer conductors. In most handheld stripping tool designs the cable will be inserted into an opening in the tool, or jaws will be closed around the cable, to position one or more cutting blades relative to the cable. The tool will then be rotated around the cable to make the necessary cuts.
One type of handheld stripping tool uses a double-edged cutting blade of the type shown in U.S. Pat. No. 3,820,420 to spirally remove material from the exterior of the cable as the tool is rotated. U.S. Pat. Nos. 5,956,852 and D297,910 show handheld coaxial cable stripping tools of this type. The tool includes an axial opening to receive the cable. One or more double-edged cutting blades extend into this opening to remove material as the cable is rotated and supported by the interior walls of the opening. Each double-edged blade includes a longitudinal cutting edge that removes material to reduce the cable diameter thereby exposing the cable interior and a transverse cutting edge that cuts approximately perpendicular to the longitudinal cutting edge to release the excess material from the adjacent exterior layer.
Double-edged cutting blades are particularly effective for larger cables where the spiral cutting action removes a small ribbon of waste material as the tool is turned. Blades of this type also work well for any cable where the outer layer of material being removed has good cylindrical rigidity so that the transverse cutting edge can cut through it as the tool is rotated. However, in some cables, such as cables having an open braided shield or a thin foil forming the outer conductor, the transverse cutting edge may crumple the foil or fail to cut all of the braid leaving a ragged transverse edge along the cut. It is particularly difficult to cut cleanly through multiple layers of thin shielding material.
Nonetheless, the double-edged cutting blade is very desirable for stripping some cables for the attachment of particular types of electrical connectors. The longitudinal cutting edge of the blade lies at the intersection of two surfaces forming a wedge that terminates in the longitudinal cutting edge. As the tool is rotated, this wedge lifts the material being removed away from the underlying material as the transverse cutting edge severs it. This lifting action protects any underlying conductive material from being nicked or damaged. The lifting action also tends to slightly separate the adjacent overlying material from the underlying material at the point where the transverse edge is cutting.
For proper connection to the coaxial cable being prepared, some electrical connectors must slide under the outer jacket or between layers of the cable. When a double-edged blade is used, the separation between the inner layer and the outer layer at the edge of the cut, provided by the described lifting action, facilitates the entry of the connector between the inner and outer concentric layers. In cables of this type, the longitudinal cutting edge, although termed a xe2x80x9ccutting edgexe2x80x9d need not actually make a severing cut, and instead serves primarily to separate layers and lift the material to be removed into position to be cut by the transverse cutting edge.
Throughout the description that follows, a blade of the type described above and in U.S. Pat. No. 3,820,420 will be referred to as a xe2x80x9cdouble-edged cutting bladexe2x80x9d even where the longitudinal edge performs only the function of separating an outer layer from an inner layer along a preexisting cylindrical shear surface between coaxial layers of the cable.
Another type of handheld stripping tool uses one or more thin planar cutting blades similar to razor blades. U.S. Pat. No. 5,713,132 shows a handheld coaxial cable stripping tool of this type. The blades are mounted on jaws that close around the cable and hold the planar blades perpendicular to the axis of the cable. Designs of his type work well on cables where the outer conductor does not have good cylindrical rigidity because the cutting force from the planar blade is oriented radially inward, instead of circumferentially as is the case with the transverse cutting edge of a double-edged blade. In the planar blade design, the jacket, outer conductor and all other material to be removed are supported against the inward cutting force of the planar blade by the underlying core of the cable.
Another advantage of the planar blade is that it is much thinner than the transverse cutting edge of a double-edged blade. This reduces the chance of tearing, crumpling or incompletely cutting the outer conductor. The planar blade extends into near-tangential contact to the inner layer being exposed. This produces a very sharp corner at the junction between the exposed inner material and the remaining outer material.
However, tools designed with planar cutting blades are not desirable for all types of cables. There is some risk of nicking the conductor below the blade if the cable is not round. The sharp corner produced between the material being removed and the material remaining is not desirable in some applications where the connector or another preparation tool must slide underneath the remaining outer material. As described above, in these applications a double-edged cutting blade produces a better cut because it lifts the outer layer slightly and spreads the remaining outer material outward in a slight bell shape at the plane of the cut.
Tools with planar cutting blades can only be used with cables where a cylindrical shear surface already exists within the cable at the desired depth so that it is only necessary to make the transverse cut. After the transverse cut is made, the outer cylinder of excess material must slide off the end of the cable, separating from the inner core being exposed along the pre-existing cylindrical shear surface.
Larger cables may have too much friction between the waste material to be removed and the inner core. The friction prevents the waste material from being removed, even if an appropriate shear surface exists. Other cables, may be too rough at the shear surface, or will need the cutting action of the longitudinal cutting edge to separate the outer material being removed from the underlying layer.
Some coaxial cables have combinations of materials and designs that make them difficult to strip with either type of tool described above.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a coaxial cable stripping tool that may be used to remove insulation from coaxial cables that are difficult to strip with conventional stripping tools.
Another object of the present invention is to provide a coaxial cable stripping tool that provides the advantages of both a double-edged type of stripping tool and a planar blade type of tool.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The above and other objects, which will be apparent to those skilled in art, are achieved in the present invention, which is directed to a tool for stripping insulation from coaxial cables. The tool includes a tool body, a double-edged cutting blade and a planar cutting blade.
The double-edged cutting blade is mounted on the tool body and includes a transverse cutting edge and a longitudinal cutting edge. The transverse cutting edge lies in a plane substantially perpendicular to an axis of the cable and the longitudinal cutting edge lies in a plane substantially parallel to the axis of the cable. As the tool is rotated relative to the cable, the double-edged cutting blade makes a helical cut and removes waste material from the outside diameter of the cable. The helical cut continues until the cable reaches a stop in the tool.
The planar cutting blade is mounted for motion relative to the tool body transversely to the axis of the cable between a cutting position and a non-cutting position. The planar cutting blade has a cutting edge that lies in a plane substantially perpendicular to the axis of the cable.
In the preferred embodiment of the invention, the planar cutting blade is mounted on a blade cassette that moves perpendicular to the axis of the cable from the cutting position to the non-cutting position. As the tool is rotated relative to the cable, with the planar cutting blade in the cutting position, it cuts down to a preset depth in the cable, typically to the inner conductor. The blade cassette can then be moved to the non-cutting position, the cable removed and the waste material can then be pulled off.
The tool body includes a notch that receives the blade cassette and supports it during cutting. The notch has opposed parallel sides that cooperate with opposed face surfaces on the blade cassette. The blade cassette face surfaces are slidingly guided between the opposed parallel sides of the notch as the cassette moves between the cutting and non-cutting positions.
The coaxial cable stripping tool also includes a guide mechanism for guiding the blade cassette during movement from the non-cutting position to the cutting position. The guide mechanism includes at least one guide rod, and preferably two guide rods, mounted to the tool body. The tool body includes corresponding guide bores receiving the guide rods. The cassette is preferably biased outwards to the non-cutting position by one or more springs. The springs may be mounted on the guide rods.
In another aspect of the invention, the tool also includes a positioning member connected to the cassette and extending through the tool body to an opposite side of the tool body from the cassette. The positioning member also may assist in guiding the motion of the cassette, but principally acts to hold the cassette during cutting so that the planar blade remains in the desired cutting position.
The positioning member is operable from the opposite side of the tool body from the cassette to move the cassette into the non-cutting position. In the design shown, the positioning member includes a barrel bolt having an enlarged head that acts as a stop to stop motion of the blade cassette at the non-cutting position. The positioning member cooperates with a spring detent mechanism to position the blade cassette in the cutting position and accurately hold the blade cassette in the cutting position as the tool is rotated to make the cut with the planar blade.
The detent mechanism preferably comprises a spring and ball detent mechanism cooperating with a detent notch formed in the barrel bolt.
In another aspect of the invention, the blade cassette and planar blade form an integral replaceable unit. The blade cassette may be formed of molded plastic, with the planar blade being positioned during the molding operation and held in the desired position relative to the blade cassette by the molded plastic.
In one embodiment of the tool, the tool body includes first and second opposed ends having corresponding first and second opposed openings for receiving the coaxial cable. The planar blade is mounted in operable position relative to the first end of the tool body and extends into the first opening when the planar blade is in the cutting position. The double-edged cutting blade is mounted in operable position relative to the second end of the tool body and extends into the second opening. In this embodiment, the planar blade makes a stripping cut when the tool is rotated while the coaxial cable is inserted into the first opening and the double-edged cutting blade makes a stripping cut when the tool is rotated while the coaxial cable is inserted into the second opening.
In an alternative embodiment of the tool, the tool body includes first and second opposed ends, the first end having a first opening therein. The planar blade is mounted in operable position relative to the first end of the tool body and extends into the first opening when the planar blade is in the cutting position. The double-edged cutting blade is also mounted in operable position relative to the first end of the tool body and also extends into the first opening. In this design, both blades are mounted near the same end and the opposite end is available for another use.